In a magnetic disk device, a slider having a magnetic head flies at a location that is extremely close to a surface of a disk by using an airflow produced by rotation of the disk. By reducing the flying height of the magnetic head, it is possible to increase the sensibility of the magnetic head and prevent unnecessary spread of a magnetic field. Therefore, reduction in flying height of a magnetic head has been required to increase the capacity of a magnetic disk device.
Recently, the flying height of magnetic heads has been reduced by improvement in design of flying surfaces and by improvement in technology of processing sliders. Furthermore, a method of actively controlling the flying height has been proposed. For example, a resistor for heating (i.e., a heater) is provided on a portion of a head and is supplied with a current to generate heat. The generated heat deforms the head so that a portion of the head protrudes toward a surface of a disk to thereby reduce the flying height. See Japanese Laid-open Patent Publication No. 5-20635.
However, if the flying height of a magnetic head is excessively reduced, the magnetic head may be in contact with a magnetic disk so as to generate vibration on the magnetic head due to a frictional force. The vibration of the magnetic head may inhibit stable recording and reading and cause abrasion of the magnetic head and the magnetic disk. The abrasion of the magnetic head and the magnetic disk may cause corrosion or discharge in the magnetic head and the magnetic disk. Therefore, it is desirable to avoid contact of the magnetic head with the magnetic disk in the magnetic disk device.
In this regard, methods of detecting contact of a magnetic head with a magnetic disk or of controlling the flying height of a magnetic head have recently been proposed by Japanese Laid-open Patent Publication Nos. 3-54780, 9-16953, 9-282601, and 2005-4909.
Generally, a magnetic disk includes a substrate, a recording layer formed on a surface of the substrate, a protective film, and a lubricant layer. While the protective film is solid, the lubricant layer forms a fluid lubrication region. Therefore, the magnetic head should be prevented from contact with the protective film more keenly than contact with the lubricant layer.
However, the techniques disclosed by the above-identified Publication Nos. 3-54780 and 9-16953 cannot distinguish contact with a protective film from contact with a lubricant layer because they merely obtain an output value of a vibration detection sensor.
The above-identified Publication No. 9-282601 discloses that a contact judgment process is performed when frequency components other than natural frequencies of an actuator and a suspension (e.g., 50 kHz to 500 kHz) are detected. The frequency components other than natural frequencies of an actuator and a suspension are equivalent to a natural frequency of an air film on the head. Thus, the frequencies used for the contact judgment do not reflect vibration caused by contact of the magnetic head with the magnetic disk. Accordingly, contact of the magnetic head with the protective film may not be detected with accuracy.
The above-identified Publication No. 2005-4909 discloses that contact with a protective film is distinguished from contact with a lubricant layer based on differences of output values from a detection sensor, i.e., differences of amplitudes of output voltages. However, since only the amplitudes of output voltages are used for the determination, vibration of a magnetic head caused by factors other than contact (e.g., a seek operation) may mistakenly be detected as contact of the magnetic head with the magnetic disk. Therefore, this method may not accurately detect contact of the magnetic head with the magnetic disk.